Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Dialpad - Switch is a cloud-based phone system built for Google Apps users.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
Sipdroid - Free and open-source SIP/VoIP Client
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs